Printed gas sensor

ABSTRACT

A printed gas sensor is disclosed. The sensor may include a partially porous substrate, an electrode layer, an electrolyte layer, and an encapsulation layer. The electrode layer comprises one or more electrodes that are formed on one side of the porous substrate. The electrolyte layer is in electrolytic contact with the one or more electrodes. The encapsulation layer encapsulates the electrode layer and electrolyte layer thereby forming an integrated structure with the partially porous substrate.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/317,222 filed Jun. 27, 2014, which is a continuation-in-part of U.S.patent application Ser. No. 13/740,327 filed Jan. 14, 2013 which is adivisional of U.S. patent application Ser. No. 12/953,672 filed Nov. 24,2010, the entire disclosures of which are hereby incorporated byreference.

TECHNICAL FIELD

The following disclosure relates generally to printed gas sensors, andin particular to a printed gas sensor comprising a liquid, polymer,solid, or gel electrolyte and a method thereof.

BACKGROUND

Electrochemical cells have been used for detection of toxic gases sincethe 1970s in, for example, fixed location instrumentation forinfrastructure (such as buildings and parking garages) and portablesafety and inspection equipment used in transportation. For example, seeStetter, J. R., “Instrumentation to Monitor Chemical Exposure in theSynfuel Industry,” Annals American Conf. of Governmental and IndustrialHygienists, 11, 225-269, (1984). These sensors may be preferred inambient monitoring applications because of their accuracy at lowdetection levels, selectivity, linearity, and power requirements.Industrial-grade electrochemical cells can cost the customer over $25each and even several hundred dollars without any electronics, even whenmanufactured in high volumes. This cost can significantly increase thecost of gas monitors and detectors, and can leave manufacturers with fewcost-effective options to create ultra-cheap, yet high performance gasdetectors. For example, high quality, accurate devices for sensingcarbon monoxide and triggering an alarm in the presence of excessiveconcentrations of carbon monoxide (CO) that may be hazardous to life orhealth are presently available for many industrial applications, butsuch devices are still too costly for use in most homes.

As a result, less expensive sensors with much lower performance arechosen to meet high volume consumer product cost goals, resulting inlower performance and a sacrifice of needed safety and health protectionfor the consumer. Additional consumer, medical, and industrialapplications will be made available with a significant reduction in thecost and dimensions of electrochemical gas sensors. Other prior art gassensors may use a liquid proton conductor where the outside surfaces ofthe sensing and counter electrodes of the sensor are coated by NAFION™layers. NAFION™ is subject to freezing at 0° C., hindering operation ofa sensor coated by NAFION™ at temperature of 0° C. and below. Further,the lifetime of these sensors can range from about 6-12 months due torapid drying of the liquid electrolyte. Thus, the sensor requiresmaintenance due to liquid electrolyte evaporation, leakage, and/orcorrosion. In addition, the sensors can have significant manufacturingcosts and be relatively large, sometimes with large electrolyte or waterreservoirs, which make integration of these sensors into modernequipment or small personal monitors difficult.

Another prior art gas sensor uses a design incorporating protonconductors, one type of electronically conductive metal catalyst for thesensing electrode, and a different type of electronically conductivemetal catalyst for the counter electrode. This allows the sensingelectrode to decompose a gas to produce protons and electrons, while thecounter electrode exhibited no activity to decompose the gas. The resultis a Nernst potential between the two electrodes, which can be used todetect a target gas. However, issues that could result from such adesign include the gas reaction being carried out slowly or interferingreactions occurring on one or the other electrode surface. Additionally,the response signal could be weak. Further, the Nernst potential may bedifficult to zero in clean air and the calibration is limited to about59 mV per decade of concentration. Again poor electrolyte or electrodestability over time can degrade performance of a potentiometric gassensor which often operate better at a high temperature.

Thus, a competitive electrochemical sensor that can cost less tomanufacture in high volume and has high performance and small size, thatwould create a new opportunity for companies to develop low-cost gasdetectors that could be manufactured in high volumes, thus making highaccuracy detectors, for example, those that monitor and detect carbonmonoxide and protect people and assets, much less expensive. This costreduction, without loss in performance, could revolutionize andtremendously expand the use of gas detectors in their application,including home carbon monoxide monitors, automobile air quality, andbuilding ventilation and controls. In addition, new applications wouldbecome possible, including safety organizations that may desire toinexpensively protect or monitor a large area from toxic gases likecarbon monoxide, and universities or scientific/environmentalorganizations wanting to study toxic gas levels over large areas. Inaddition, an electrochemical sensor that also can be small can be usedin cell-phones to enable worldwide networks of CO and other gasmonitors.

The traditional porous, composite electrode is comprised of 10-40%polytetrafluoroethylene (PTFE) by weight and 60-90% catalyst by weight.The traditional electrode has possible residual dispersing, surfactantsand thickening agents. These residual components are chemically inertand electrochemically inert. These electrodes are cured and/or sinterednear the melting point of PTFE, typically 290-310 C. This requiresprinting on substrates such as porous PTFE that can withstand the PTFEcure temperatures. The PTFE serves as a binder to hold the catalystparticles together in a porous bed. It also serves as the hydrophobicportion of the composite bed electrode to provide a proper environmentfor a triple-phase boundary. This triple-phase boundary is desirous forgas-phase amperometric sensors.

While a variety of devices and techniques may exist for detecting gases,it is believed that no one prior to the inventors has made or used theinventive embodiments as described herein which have allowed the thinand tiny form factors and the low cost assembly achieved herein togetherwith the high performance.

SUMMARY

In one example, a printed gas sensor is disclosed. The sensor maycomprise: a substrate that is at least partially gas porous or gaspermeable; an electrode layer, wherein the electrode layer comprises twoor more electrodes, with one at least partially porous electrode, thatare formed on one side of said porous substrate; a solid, liquid, gel orsimilarly functional electrolyte layer, wherein the electrolyte layer isin electrolytic contact with the electrode layer, and an encapsulationlayer, wherein the encapsulation layer encapsulates the electrode layerand part or all of its substrate and electrolyte layer, thereby formingan integrated structure with the porous substrate.

In another example, a printed gas sensor is disclosed that may comprise:a porous substrate; an electrode layer, wherein the electrode layercomprises two or more porous electrodes that are formed on one side ofsaid porous substrate; a wicking layer formed on the electrode layer; asolid, liquid, or gel electrolyte layer, wherein the electrolyte layeris in electrolytic contact with the two or more electrodes; and anencapsulation layer, wherein the encapsulation layer encapsulates theelectrode layer, wicking layer and electrolyte layer thereby forming anintegrated structure with the porous substrate.

In still another example, a method for manufacturing a printed gassensor is disclosed. The method comprises printing two or moreelectrodes with one at least partially porous electrode onto one side ofthe at least partially porous substrate using a metal catalyst ink;curing the porous substrate; bonding an optional encapsulation layerhaving a capillary channel to the porous substrate thereby encapsulatingthe two or more porous electrodes and forming an electrolyte reservoir;filling the electrolyte reservoir through the capillary channel with aliquid or gel electrolyte; and sealing the capillary channel.

In a further example, porous gas electrodes comprise alternative polymercomponents replacing the standard PTFE aqueous dispersion particles. Insome examples, the standard PTFE particles are replaced with dry PTFEparticles. In other examples, the standard PTFE particles are replacedwith polypropylene or polyethylene particles.

While the specification concludes with claims, which particularly pointout and distinctly claim the invention, it is believed the presentinvention will be better understood from the following description ofcertain examples taken in conjunction with the accompanying drawings.

In the drawings, like numerals represent like elements throughout theseveral views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary version of a printed sensor in cross sectionview.

FIGS. 2A & 2B depict a substrate layer of the exemplary printed sensorshown in FIG. 1.

FIGS. 3A & 3B depict an encapsulation layer of the exemplary printedsensor shown in FIG. 1.

FIG. 4 depicts an exemplary version of a printed sensor in cross sectionview.

FIG. 5 depicts a substrate layer of the exemplary printed sensor shownin FIG. 4.

FIG. 6 depicts a substrate and electrode layer of the exemplary printedsensor shown in FIG. 4.

FIG. 7 depicts an encapsulation layer of the exemplary printed sensorshown in FIG. 4.

FIG. 8 depicts an exemplary electrochemical reaction in the porous gaselectrode.

FIG. 9 depicts an exemplary version of a printed gas sensor.

FIG. 10 depicts an exemplary version of a printed gas sensor.

FIG. 11A depicts an exploded view of the printed gas sensor of FIG. 9.

FIG. 11B depicts a partially assembled view of the printed gas sensor ofFIG. 9

FIG. 12 depicts an exemplary version of a sensor layer of a printed gassensor.

FIG. 13 depicts an exemplary version of a sensor layer of a printed gassensor.

FIG. 14 graphically depicts the response signal of electrodes comprisingdry PTFE particles and electrodes comprising a PTFE aqueous solution.

FIG. 15 graphically depicts the response signal in μAmps of electrodesformed with 5-7 μm PPRO powder particles.

FIG. 16 graphically depicts the response signal in μAmps of electrodesformed with 2-4 μm PE powder particles.

FIG. 17 graphically depicts results of CO signal vs. polymer particlesize.

FIG. 18 graphically depicts a relationship between gas port size andsensor to sensor consistency.

The drawings are not intended to be limiting in any way, and it iscontemplated that various embodiments of the invention may be carriedout in a variety of other ways, including those not necessarily depictedin the drawings. The accompanying drawings incorporated in and forming apart of the specification illustrate several aspects of the presentinvention, and together with the description serve to explain theprinciples of the invention; it being understood, however, that thisinvention is not limited to the precise arrangements shown.

DETAILED DESCRIPTION

The following description of certain examples should not be used tolimit the scope of the present invention. Other features, aspects, andadvantages of the versions disclosed herein will become apparent tothose skilled in the art from the following description, which is by wayof illustration, one of the best modes contemplated for carrying out theinvention. As will be realized, the versions described herein arecapable of other different and obvious aspects, all without departingfrom the invention. Accordingly, the drawings and descriptions should beregarded as illustrative in nature and not restrictive.

Examples described herein include a printed gas sensor comprising: aporous substrate; an electrode layer, wherein the electrode layercomprises two or more electrodes that are formed on one side of saidporous substrate; a liquid or gel electrolyte layer, wherein theelectrolyte layer is in electrolytic contact with the two or moreelectrodes; and an encapsulation layer, wherein the encapsulation layerencapsulates the electrode layer and electrolyte layer thereby formingan integrated structure with the porous substrate. The printed gassensor may be used to detect and measure a wide range of target gaseouscomponents. By way of example only, it can be used to detect CO, H₂S,NO, NO₂, SO₂ O₃, and related compounds that can be eitherelectro-oxidized or electro-reduced compounds. For exemplaryelectro-oxidized and electro-reduced compounds see: Stetter, J. R.Sang-Do, Han, and G. Korotchenkov, “Review of Electrochemical HydrogenSensors,” Chemical Reviews 109(3), 2009, pp 1402-1433; Joseph R. Stetterand Jing Li, in Modern Topics in Chemical Sensing: Chapter 4,“Amperometric Gas Sensors—A Review,” Chemical Reviews, 108 (2), 2008, pp352-366; Chang, S. C., Stetter, J. R., Cha, C. S., “Amperometric GasSensors”, Talanta, 40, No. 4, pp 461-467, (1993).

The porous substrate is gas permeable at least in part and has porositysufficient to allow a gas sample to permeate through and react at theworking electrode which is also sufficiently porous to allow the sampleto diffuse to the metal surface and react. In one exemplary embodiment,when an aqueous or hydrophilic room temperature ionic liquid (RTIL)electrolyte is utilized, the porous membrane is selected fromhydrophobic membranes. In an alternative embodiment, when a hydrophobicorganic electrolyte (i.e. an ionic liquid or more particularly an RTIL,a salt in the liquid state that primarily comprises ions and short-livedion pairs) is utilized, the porous membrane is selected from oligophobicmembranes. This can be measured by the contact angle. In some preferredembodiments, the contact angle of the RTIL/organic electrolytes on thechosen membrane is the same or greater than the contact angle for wateror sulfuric acid (about 90°) on the chosen membrane. Exemplary membranescan be hydrophobic or hydrophillic. Exemplary porous hydrophobic andoligophobic membranes include PTFE (e.g., MuPor™ by Porex™ and Zitex™ bySaint-Gobain™), polypropylene (e.g. polypropylene filters by Pall™,polypropylene membranes by Sterlitech™), polycarbonate (e.g.,polycarbonate track etch (PCTE) membrane disc filters by Sterlitech™),and PVDF (e.g., Immobilon™ by Millipore™). Exemplary porous hydrophillicmembranes include polyethersulfone (e.g., polyethersulfone membranes byPall™), surface modified PVC (e.g., PVC with ozone induced graftpolymerization), and surface modified polypropylene (e.g., polypropylenewith UV radiation). Exemplary membranes can also be made by treating aporous membrane with cytop to make the porous membrane hydrophobic,derivatizing a surface of a porous membrane with silane to make thesurface hydrophobic, or selecting surface treatment chemistry with adesired level of hydrophobicity or oligophobicity.

Embodiments of a printed gas sensor described herein can utilizeelectrolytes, such as RTILs or H₂SO₄ having certain contact angles.Ranges of working contact angles allow a generalization as to which typeof RTILs or other electrolytes can be chosen based on the range ofcontact angles. These contact angles are important for viable gas sensorperformance of the membrane of the printed gas sensor. The contactangles of exemplary RTIL electrolytes listed below are contact anglemeasurements of a 2 μL droplet of each exemplary electrolyte on MuPorporous PTFE. 4M H₂SO₄ has about a 118° contact angle,1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide has abouta 99° contact angle, 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide has about a 106° contact angle,1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide has abouta 90° contact angle, 1-ethyl-3-methylimidazolium ethyl sulfate has abouta 113° contact angle, 1-butyl-3-methylimidazolium tetrafluoroborate hasabout a 139° contact angle, 1-ethyl-3-methylimidazoliumtetrafluoroborate has about a 122° contact angle,1-butyl-1-methylpyrrolidinium dicyanamide has a contact angle betweenabout 131° and 134°, and 1-butyl-1-methylpyrrolidiniumbis[(trifluoromethyl)sulfonyl]imide has about a 71° contact angle. Insome embodiments of the printed gas sensor, it is desirous to have anelectrolyte comprising an RTIL having a contact angle greater than 115°.This provides a high quality response in measurements of hydrogensulfide and ozone and should be suited to other gas measurementchemistries.

In one exemplary embodiment, the substrate is porous PTFE [e.g. tradenames Zitex™ Gortex™, or MuPor™] and the electrode metal material isapplied as an ink composite to make a porous gas diffusion electrode.Further, the porous or partially porous substrate may comprise porouspolymeric materials, such as porous polytetrafluoroethylene, porouspolyethylene, porous polypropylene, porous polyisobutylene, porouspolyester, porous polyurethane, porous polyacrylic, porous fluorinepolymer, porous cellulosic polymer, porous fiberglass that may betreated to alter the hydrophobicity or oligophobicity, any other porousnon-reactive thermoplastic, or composites or mixtures thereof. In someembodiments, the porous or partially porous substrate is non-wettable.The porous substrate thickness can range typically from about 100microns to 250 microns and may include pores having diameters in therange of about 0.1 micron to about 5 microns. The porous substrate canbe any thickness and can have any pore diameter that creates theappropriate porosity. The porous substrate can be placed on a backplate220 or support with a controlled-size hole [see FIG. 5] that allows theappropriate gas access into the sensor's working electrode. Thethickness of this backplate or support/porous substrate can be 0.002 to0.005 inches and can comprise polyethylene terephthalate (PET),polycarbonate, polypropylene or other suitable plastic as shown in FIGS.4 and 5 but any thickness to create the selected or desired gas accessis acceptable. The two layers can be laminated, glued or otherwiseplaced and held adjacent to one another. In another embodiment, thebackplate access hole provides the porous entry and obviates the needfor a separate and distinct porous or partially porous substrate.

The porous substrate can have an access port for entry of a gas sampleto be measured. The access port should be sized to allow a gas sample toenter the sensor, but not too large such that supply of the gas sampleis beyond the capacity of the electrodes or the desired range ofreactivity. For example, the access port hole may range in size fromabout 0.003 inches in diameter to about 0.080 inches in diameter. Thelarger hole can be desired for lower concentrations (e.g. a 1-10 ppm COsensor) while the smaller hole can be chosen for a broader range sensor(e.g. a 0-10,000 ppm CO sensor). The access port may also be larger orsmaller and depend upon the gas to be detected, the range of the sensordesired, and the particular construction and need of the sensingreaction and sensor mechanism of response (e.g., diffusion limitedsignal, or reaction rate limited signal, or other limitation of theoverall electrochemical sensor). The access port may be formed bystamping, laser cutting, die cutting, drilling, or other known exemplaryprocesses. The access port may be a single porous area or hole or acollection of pores or holes or materials that have gas permeability toallow the analyte to enter. The access port may contain a reactivematerial to selectively allow the analyte to pass and retain certainunwanted interferences. In operation, a gas sample may enter through theaccess port and pass through the, sometimes optional, porous substrateon the side opposite of the electrode layer in order to reach theworking electrode and react. Further, the gas sample may permeatethrough the access port. The access port can be a straight through holeas shown in the figures or a tortuous path with or without obstructionsor filter materials. This allows gas access of the analyte to theworking electrode electrolyte interface.

The access port may be covered by a filter, which is adhered to orotherwise held in place next to the access port of the substrate. By wayof example only, the filter can comprise any material that protects theelectrode layer from poisoning or clogging particles or any otherunwanted direct exposure to the environment from which the targetanalyte (that which is to be analyzed) originates. By way of exampleonly, the filter can also comprise any material that can remove theeffects of wind and dust, evaporation of the electrolyte from the sensorand reduce effects of pressure fluctuations and air turbulence on thegas sensor. By way of example only, the filter can also comprise anymaterial that may remove interferents, for example, hydrogen sulfide ina CO gas sensor, so that a target gas, CO in this example, may passthrough to the electrode layer unimpeded. Examples of a filter caninclude porous polytetrafluoroethylene (PTFE), carbon, impregnatedcarbon cloth, KMnO₄ on alumina, and reactive material in the form ofpowder or composite and a tortuous path. Filters for NO can includetriethanolamine on a silica support. As one skilled in the art willappreciate, there are other chemistries that can be developed for gassensing for selective filters based on acid-base and other absorptive orreactive and other chemistries. For example, Cu-Acetate, bicarbonate, orsimilar basic salts can be used to remove acid gases like H2S or SO2.For ammonia removal, an acid media such as an acid washed aluminasubstrate can be used; however, this can remove basic gases. The mediashould be dispersed in order to remove the gases efficiently withoutimpeding the gas sample flow to the sensor.

The electrode layer comprises two or more electrodes that may bedesigned to provide a high or low surface area to control theelectrode-electrolyte interface and maximize the current output of thesensor and minimize the noise in the sensor. The optimum analyticalsignal for the target analyte will be a combination of considerations ofsignal, background, noise, and interferences. The electrode layer canact as a gas-permeable membrane and provide a physical boundary betweenthe electrolyte and the gas. This physical structure of the workingelectrode is important to control and this control is offered by controlof the ink formulation and curing process. The electrode material canalso be sputtered or physically or chemically deposited onto or made tolie next to the substrate layer. The electrode layer may be formed onone side of the porous substrate by screen printing or inkjet printing.The thickness of the electrode layer may range from about 100 nanometersto about 125 microns (0.005 inches or 5 mil) typically but of course canbe any thickness that is effective at reacting the analyte. Screenprinting typically produces layers that are 0.001-0.005 inches dependingon the ink formulation and the screen mesh size used to deposit thecomposite material. In one exemplary embodiment, screen printing isattractive because it is a fast, efficient process and can printmultiple electrodes at the same time (i.e. the two or more electrodesneeded for sensor operation) and multiple devices on a large substratearea, simultaneously. The materials for the screen printing can be anyrange of materials needed for the electrode including Pt particles forCO sensors and Au particles for H₂S sensors and SWCNT (single walledcarbon nanotubes) for Ozone sensors.

The electrode layer only requires two electrodes but more than two arepossible. A first electrode may be referred to as the sensing or workingelectrode and comes into contact with a target gas sample that is to bedetected. A second electrode may be known as the counter, auxiliary,counter-reference, or common electrode. When the target gas to bedetected comes in contact with the sensing electrode, an oxidation orreduction reaction takes place at the sensing electrode, with acorresponding reduction or oxidation reaction occurring at the counterelectrode.

For example, in the case of a carbon monoxide gas sensor, the followingoxidation/reduction reaction may occur. Carbon monoxide is oxidized asfollows:CO+H₂O→CO₂+2H⁺+2e ⁻  (1)

In the corresponding reduction reaction, the protons (hydrogen ions)migrate across a proton conductive electrolyte membrane to a counterelectrode where they may react with oxygen as follows:2H⁺+2e ⁻+½O₂→H₂O  (2)

It may be desirous to include an additional or third electrode with aconstant or almost constant potential throughout the reaction. Such anelectrode may be called the reference electrode and can play a role instabilizing the potential of the sensing electrode. Alternatively, thecounter electrode may be non-polarizable and act as a referenceelectrode. The sensor is interfaced with suitable electronic componentsto read out the current as a measure of the reacting gas concentration.Further, if the currents in the sensor are small enough with respect tothe size of the sensor electrodes to minimally polarize the counterelectrode, then the counter electrode can be used as a referenceelectrode in a three electrode circuit. These suitable electroniccomponents can non-exhaustively include circuits such as current tovoltage convertors, potentiostats, amperostats, and current mirrors. Agalvanic sensor operation and circuit is also possible in some cases.

The electrode layer may typically comprise from about 60% to about 90%of a metal catalyst, from about 2% to about 40% of polymer (e.g.,micron-sized Teflon particles), in an ink formulation that contains lessthan about 10% of an optional binder, less than about 10% of an optionalsurfactant and from about 0% to about 10% of one or more optionalmodifiers. The binder is designed to remain in the electrode and providestructural support while the modifier can be an additive that altersproperties of the electrode such as wetting character or porosity. Themetal catalyst may be a powder and, by way of example only, may comprisePt, Pd, Au, Ag, Ru, Rh, Ir, Co, Fe, Ni, C, or other noble or reactivemetals, and alloys or admixtures thereof. The metal catalyst may be acarbon supported catalyst. For example, the carbon support may benanoparticulate carbon, ball-milled graphitic carbon, single walledcarbon nanotubes, Au nanoparticles, or any suitable support. The bindercan assist in providing the ink formulation with a proper viscosity andvaporization/drying rate for screen printing and/or function to hold theink to the substrate and merge with the substrate during the curingprocess to control electrode properties such as hydrophobicity,hydrophilicity and/or porosity (amount and type). Examples of suitablebinders include Nicrobraz-S (available from Wall Colmonoy Corporationlocated in Madison Heights, Mich.), or solutions of polyvinyl alcohol(PVA). Other suitable binders include silicate or aluminate materials,or polymers such as ethyl cellulose. The surfactant can act as thesolution stabilizer for the ink composite and may comprise solvents,such as water, triton-100, carbopol or other material. The modifierscomprise small amounts of additives, which can be active in controllingthe behavior of the inks before, during, and/or after processing andcuring. Suitable modifiers may include polyvinyl alcohol, 1-propanol,gum arabic, sodium n-dodecyl sulfate, ethanol, or a composite material.The materials used in the ink composition should generally evaporate orbake out of the composite electrode during a curing process, or thematerials should be electrochemically inert and not alter electrodeperformance, porosity, or wettability in an intended way. The materialsused in ink composition should generally leave behind an electrodecatalyst of the desired porosity, chemistry, density, and hydrophobicityor hydrophilicity for optimum interaction with the electrolyte andanalyte gas. Other suitable components for this ink formulation will beapparent to those of ordinary skill in the art in view of the teachingsherein on controlling the proper chemical and physical properties ofelectrodes for gas sensing.

The electrode layer is in electrolytic contact with the electrolytelayer. The electrolyte layer can be any suitable material capable ofproviding the needed electrolytic system for the sensing and counterreactions and interface to the electrodes. Suitable electrolytes includeaqueous systems of acids, bases, and salts as well as polymerelectrolytes like Nafion, or non-aqueous systems like propylenecarbonate lithium perchlorate, polyethylene oxide lithium chloride, orionic liquids. The electrode layer may also be in electricalcommunication with a measurement device, such as a potentiostat circuit.Electrical communication may comprise the electrode layer having tracksscreen printed directly on the substrate, such as printed runners andconductive traces, which connect the two or more electrodes to apotentiostat circuit. The electrode layer may be connected to thepotentiostat circuit exterior to the gas sensor through the tracks.Electrical communication may also comprise wire connections running fromthe electrodes through the gas sensor to the potentiostat circuit.

In some embodiments, the electrode connections run from the active areaof sensing at the working electrode inside the sensor to outside thesensor through a path that is sealed to any material flow. Such aconfiguration minimizes and/or eliminates electrolytes, ions, gases,liquids, or solids of any kind transporting across the seal. The printedrunner is comprised of a conductor through which electrons can flow butmaterials cannot flow, such as, for example, a conductive trace havingconductive ink. A printed runner's conductor component may include asolid wire or ribbon comprising a noble metal. For example, the printedrunner can comprise a pressure sensitive adhesive (PSA) with a conductorcomponent. Exemplary noble metals include but are not limited to Pt, Au,Ru, or Ir. In some embodiments, the printed runner comprises a carbonrunner that is non-porous and non-wettable with respect to theelectrolyte. The printed runner includes a polymer adhesive that can bea welded polymer, a PSA, or another thermoset or UV cure that is inertwith respect to the elements contained within the sensors. The adhesiveseals to the carbon runner which is electronically conductive. It isdesired that the sensor maintain this seal over the lifetime of thesensor, which can be months to decades.

The electrolyte layer may be a liquid or gel or solid or composite andis in electrolytic contact with the two or more electrodes of theelectrode layer. In some embodiments, the electrolyte layer formselectrolytic contact with the electrode layer by impregnating theelectrode layer, such as, for example, impregnating a wick that includestwo or more electrodes coupled to the wick. By way of example only, theelectrolyte layer may comprise phosphoric acid, sulfuric acid, aqueousphosphoric acid, aqueous sulfuric acid, methanesulphonic acid, aqueousphosphate salt solution, aqueous sulfate salt solution, potassiumhydroxide, aqueous potassium acetate, lithium perchlorate in propylenecarbonate, polyvinyl alcohol with sulfuric acid, polyacrylic acid, anionic gel electrolyte, or ionic liquid. The electrolyte layer maycomprise any suitable charge carrying entity that will also support thedesired electrochemical reactions in the sensors and not createundesired reactions or conditions. Other suitable materials will beapparent to those of ordinary skill in the art in view of the teachingsherein.

The electrolyte layer may have a substantially uniform thickness in theprinted sensors, typically from about 1 mil to about 5 mils (125microns). The electrolyte layer may include a matrix or gelling agent toprevent dryout or movement during vibration or use or otherwise enhancesensor properties. The electrolyte layer can cover at least a part ofthe sensing electrode area and the counter electrode in the sensor. Thatis, it is in electrolytic contact with the electrode layer, generallyvia an electrolyte reservoir formed on at least part of the electrodelayer. The electrolyte layer may cover the entire electrode layer or theelectrolyte layer may cover part of the electrode layer. The electrodelayer may include a chamber adjacent to it to contain additionalelectrolyte or supporting material to enhance the lifetime or otherperformance of the electrolyte. In some embodiments, the electrode layerand the electrolyte layer are co-located such that the one or moreelectrodes are disposed within the electrolyte layer to facilitateelectrolytic contact between the electrolyte and the one or moreelectrodes.

The encapsulation layer encapsulates the electrode and electrolyte layerforming an integrated structure with the porous substrate. Theencapsulation layer essentially forms a housing structure with theporous substrate defining an internal region that comprises an electrodeand electrolyte layer through which no material may enter or leaveexcept through one or more access holes. The one or more access holesare designed to allow analyte access into and out of the housingstructure. No material (except gases through the gas access port) canescape the inside of the sensor. This includes electrodes andelectrolytes. The encapsulation layer may comprise polyimide,polycarbonate, polyethylene, polypropylene, polyisobutylene, polyester,polyurethane, polyacrylic, fluorine polymer, cellulosic polymer,fiberglass, polytetrafluoroethylene, any other non-reactivethermoplastic. The encapsulation layer may also comprise pottingcompounds, other materials or mixtures or composites thereof that can besuitably bonded to form the encapsulation. Other suitable materials willbe apparent to those of ordinary skill in the art in view of theteachings herein. The encapsulation layer may have a substantiallyuniform thickness of from about 0.002 to 0.015 inches (2-15 mils) or anysize that allows sufficient encapsulation of the sensor. The thicknessand nature of the encapsulation and its placement depend upon the sizeof the sensor, the design of the sensor, and the processes used in theassembly.

The encapsulation layer may further comprise a capillary channel forentry into the electrolyte layer. Such design can expedite electrolytefilling and sensor assembly. The encapsulation layer may furthercomprise a gas vent hole, which allows air to exit the electrolytereservoir as the electrolyte fills the reservoir. The gas vent hole mayalso allow venting in applications where there are large pressurefluctuations, for example, gas detection on airplanes or in submarines.By way of example only, the capillary channel and gas vent hole may beformed using plastic film stamping operations, laser cutting or diecutting to create contours and/or holes. The encapsulation should formand not close the gas access design to allow analyte entry as discussedabove.

The printed sensor may further comprise a wicking layer that can serveas a separator or absorbent layer between the electrode layer andelectrolyte layer. It can also serve as a material to wick theelectrolyte into the sensor during production and hold the electrolyteagainst the electrodes. The wicking layer may be screen printed orinkjet printed onto all or a part of the electrode layer. The wickinglayer may comprise silicates, silicon carbide, carbon, graphite,alumina, fiber glass, polymer, or other inert materials that can form aporous wick. The wicking layer may have a substantially uniform orvariable thickness of from about 5 to 125 microns in the sensors hereinbut can be any suitable thickness that allows functionality of thewicking layer. In some embodiments, the electrolyte can impregnate thewicking layer to facilitate electrolytic contact between the one or moreelectrodes and the electrolyte. In some embodiments, the printed gassensor does not include a wicking layer and the one or more electrodesare screen printed directly onto the porous substrate.

In some embodiments, the wick is not electrically conductive but formsan ionically conductive connection between the electrodes when floodedwith ionically conductive electrolytes. When a reaction occurs at theelectrode, the electrons and ions created in the electrochemicalreaction are conducted by an electrolyte solution. The electrolytesolution can be free flowing or encompassed in a wick. Accordingly, inthis exemplary embodiment, the wick is not ionically conductive, but thewick plus electrolyte structure in the sensor has ionic conductivity.

In some embodiments, the printed gas sensor is manufactured by thefollowing method: printing two or more porous electrodes onto one sideof a porous or partially porous substrate, curing the porous substrate,and bonding an encapsulation layer having a capillary channel to theporous substrate which encapsulates the two or more electrodes and formsan electrolyte reservoir. The method continues by filling theelectrolyte reservoir with electrolyte through the capillary channel andsealing the capillary channel. In another embodiment, the printed gassensor is manufactured by the following method: printing two or moreporous electrodes onto one side of a porous or partially poroussubstrate, curing the porous substrate, printing or placing anelectrolyte in the electrode area, and then bonding an encapsulationlayer to the porous substrate, thereby encapsulating the two or moreelectrodes with electrolyte and forming an electrolyte reservoir. Atthis point the electrolyte reservoir is full. The method continues bysealing the entire chamber of electrodes and electrolyte with the gasaccess port being the only route for gas access. In further embodiments,the electrolyte reservoir can have a vent formed in it for exceptionalpressure change applications. In each embodiment, the electrolyte andelectrode chambers become sealed so that no electrode or electrolytematerial can escape the encapsulation except through the access port inthe same manner as analyte enters the printed gas sensor. In eachembodiment, the access port can be sealed.

The method may further comprise forming a substrate layer frompolytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyester, polyurethane, polyacrylic, fluorine polymer, cellulosicpolymer, fiberglass, a mixture thereof, or any other non-reactivethermoplastic or bondable polymer. The substrate layer may be formed bylaser cutting, die cutting, stamping, roll milling, or other filmforming processing.

The method may further comprise forming an encapsulation layer frompolytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyester, polyurethane, polyacrylic, fluorine polymer, cellulosicpolymer, fiberglass, a mixture thereof, or any other non-reactivethermoplastic polymer or inert single or dual mixture epoxy typeencapsulation compounds. The encapsulation layer may be formed bymolding, laser cutting, die cutting, stamping, or other suitableprocesses.

An ink composition is used to print the two or more electrodes onto oneside of a porous substrate. As mentioned above, the ink composition usedto print the electrode layer (i.e., two or more porous electrodes) maycomprise: from about 60% to about 90% of a catalyst, from about 10% toabout 50% of polymer particles (e.g., micron-sized Teflon particles),less than about 10% of a binder, less than about 10% of a surfactant andfrom about 0% to about 10% of one or more modifiers. The catalyst cancomprise noble metals like Pt or Pd or alloys or supported catalystslike Pt on carbon or other supports as such formulations are well knownin the art. Printing may be done by screen printing, gravure, inkjetprinting, stenciling, or other suitable printing technology known in theart that may be used in accordance with the teachings herein. In ascreen printing process, the printing screen may comprise stainlesssteel wire, a plastic mesh, or a platinum mesh across a screen frame.The mesh size may vary depending upon the desired print film thickness,and hence the thickness of the printed two or more electrodes. The areato be screen printed may be patterned on the screen using a desiredelectrode design template. A squeegee may then be used to spread inkover the screen and the desired pattern is printed on a poroussubstrate. In an inkjet printing process, it is desired that the formulaof the ink be controlled so that the ink remains printable for some timeon the screen and does not dry too fast. The added retardant shouldcontrol this process and one exemplary embodiment herein uses aPVA/water solution as the retardant or modifier additive to themetal-plastic ink composite formula.

The method may further comprise a curing step where the sensor ink andporous substrate may be cured by heating or drying at about 100° C.-200°C., such as, for example at 150° C. for about 10 minutes in an oven.Further curing when using PTFE may require additional heating to about280° C. to 300° C. for about one hour to form a suitable cured andporous diffusion electrode for gaseous reactions. The curing step may beused to remove any solvent present. Remaining solvent not removed duringthis step may cause problems by polluting the atmosphere surrounding thesensor element. Curing may be performed by, for example, forced airdrying (e.g., at elevated temperatures), infrared irradiation,ultraviolet irradiation, ion-beam irradiation, gamma irradiation, andcombinations thereof. The curing step is chosen to fix the electrodestructure in the remaining composite porous or partially porouselectrode. In one exemplary embodiment for CO, for example, the mixtureof PTFE particles (30%) and Pt particles (60%) and ethylcellulose (10%)is cured at 300° C. for one hour to create a CO electrode having auseable porosity and hydrophilicity/hydrophobicity for measurement of COin air at 0.1-10,000 ppm. Other electrode embodiments will be describedin more detail below, including electrodes formed from a PTFE catalystink suspension, dry PTFE powder, polypropylene powder, or polyethylenepowder. Other exemplary electrodes formulations include ink preparedwith 3.125 wgt % ethyl cellulose plus 1.25 wgt % PVA.

The method may further comprise printing a wicking layer onto the two ormore electrodes and/or porous substrate. Printing of the wicking layermay be accomplished by screen printing or inkjet printing using a slurryof ink comprising particulate wick material. As mentioned above, theparticulate wick material may comprise silicates, silicon carbide,carbon, graphite, fiberglass, fiberglass filter paper, or other porousmaterials or fibrous materials that when matted together form pores orwickable substrates. In these embodiments, the wick materials should notbe electrically conductive. The wicking ink may also comprisesilica-based filter paper that is ground or mashed into small particlesand mixed with water or water with an added salt, acid or base. Thecomposition has sufficient water and vehicle (e.g. PVA, ethyl celluloseor other retardant) to make an ink suitable for printing.

In some embodiments, the wicking layer may also include an electrolytematerial such as a salt (e.g., H₂SO₄), acid, base, or the like. Theelectrolyte material can be a dry material and can be activated into aliquid electrolyte through exposure to a solvent, such as water vapor.The dry electrolyte material can allow the printed gas sensor to beassembled dry, for example in a dry box, without the need for wetelectrolyte addition. Any gas access areas (holes, porous regions, orthe like) can be covered by a removable adhesive, such as a sticker, toseal the printed gas sensor from any gas entry. When the electrolytematerial is exposed to air, the vapor in the air can enter the printedgas sensor and mix with electrolyte material to fill the printed gassensor with liquid electrolyte. This can be accomplished by removing theremovable adhesive, for example, when the printed gas sensor ispositioned for use.

The bonding of an encapsulation layer or substrate layer may comprisethermal bonding, chemical bonding, adhesive bonding, ultrasonic bonding,lamination, pressure bonding, o-ring bonding or welding. Once theencapsulation layer is bonded to the porous substrate thereby forming anelectrolyte reservoir, the electrolyte reservoir is filled. Filling maybe done by submerging the capillary channel or the entire printed sensorin electrolytic solution. As the capillary channel or printed sensor issubmerged, electrolytic solution rises through the capillary channel andinto the electrolytic reservoir. Once inside the reservoir, the wickinglayer, if present, may capture some of the electrolyte thereby holdingit against the electrode. After the reservoir is filled, the capillarychannel and any vent holes may be sealed by, for example, thermalsealing, chemical sealing, adhesive sealing (e.g., epoxy). When wickingis used, a vent is desired for the air to flow out and this should alsobe sealed at the end. This seal can be potting materials or a gas porousvent to be used for equalization of pressure when the sensor is used forpressure change applications.

The resulting printed gas sensor may be of various sizes and dimensions.For example, the thickness of the printed gas sensor may be less thanabout 1000 micron (about 1 mm). In embodiments of the printed gas sensorfurther comprising a filter layer and a reservoir overflow layer, thethickness is about 3000 microns (about 3 mm). We have constructedprototype sensors where the total thickness of all layers is about 500microns thick. Current prototypes have a surface area of about 1 mm² toabout 9 mm² and a thickness of about 0.5 mm to 1 mm. Because silk screenresolution is continuously improving, smaller sensors may be possiblewith the approach described herein.

The gas sensor performance of the exemplary CO electrodes demonstratedherein may have a measurement range of from about 0 ppm to about 1000ppm CO. In another example, the gas sensor performance for CO may have ameasurement range from about 0 ppm to about 500 ppm. The measurementoutput signal from the screen-printed CO electrode in the gas sensor mayrange from about 1 nanoamps/ppm to about 25 nanoamps/ppm. Themeasurement output signal from the gas sensor may also range from about10 nanoamps/ppm to about 25 nanoamps/ppm. These signals are rangeselectable by choosing the size of the gas access port and can bereduced with smaller holes for gas access. Even smaller signals (e.g.,10-100× smaller) are possible and can be amplified using currentlyavailable electronics. The printed gas sensor may produce a measurementresponse time of less than about 30 seconds at 20° C. The printed gassensor may also produce a measurement response time of less than about20 seconds at 20° C. Thinner sensors can produce faster response timesbecause diffusion times decrease over shorter diffusion distances. Forexample, electrolyte volumes herein still produce a very stable sensoreven under conditions of near 0% RH for a long time giving this printedsensor a long lifetime of CO monitoring in field use.

In one exemplary embodiment for CO monitoring, the printed gas sensorelectrochemically senses gases using an electronic circuit called apotentiostat. In the potentiostatic method or mode, the gas to bequantitatively sensed or detected may contact the sensing or workingelectrode in the presence of an electrolyte and generate a current thatis proportional to the amount of target gas entering the cell. Thecommon or counter electrode can run the complementary half-cellreaction. The reference electrode operating in a potentiostatic circuitcan maintain the thermodynamic potential of the working electrode duringsensing. Simple electronics may be interfaced to the sensor to convertthe current to a voltage and amplify the voltage for meter or recorderreadout.

Turning to the figures, FIGS. 1-3 depict an example of a printed gassensor operable to detect and measure carbon monoxide levels. Of course,other target gases that may be detected and/or measured using theprinted gas sensor will be apparent to those of ordinary skill in theart in view of the teachings herein. Further, while exemplary geometrieshave been disclosed, other geometries will be possible to those ofordinary skill in the art. In addition, while exemplary combinations ofink properties (viscosity, composition, drying rate, etc.) and inkstructure (thickness, porosity, etc.) have been disclosed for theexemplary sensors, one of ordinary skill in the art can envision otherinks and compositions and properties and structures. While exemplarycombinations of these specially designed parts with the printingprocesses and assembly processes have been disclosed, alternatives willbecome obvious to those skilled in the art for the design and assemblyof a family of related electrochemical gas sensors by this new approachthat combines the many special features of materials and processesdescribed herein.

Referring now to FIG. 1, an exemplary version of printed gas sensor 100is shown. The printed gas sensor 100 shown comprises: a porous substrate200, an electrode layer 300, a liquid or gel electrolyte layer 400 inelectrolytic contact with the electrode layer 300, and an encapsulationlayer 500 that encapsulates the electrode layer 300 and electrolytelayer 400 thereby forming an integrated structure of a printed gassensor 100 with the porous substrate 200. It should be understood thatFIG. 1 is a schematic depiction of the printed gas sensor 100 and is notto scale. For example, the electrode layer 300 is not severed by theencapsulation later 500. In some embodiments, passes underneath theencapsulation layer 500 and is partially pushed into the surface of theporous substrate 200. In other embodiments, encapsulation layer 500 issealed to electrode layer 300 to create a fluid tight seal. In thisexample, the substrate layer 200 comprises porouspolytetrafluoroethylene, though other porous materials may be used withporosity desirably in the region 205 for gas access, and has fourelectrodes shown printed onto one side of the porouspolytetrafluoroethylene substrate 200. If porous PTFE is used, theporosity can be filled with adhesive or a back-plate or similar approachas long as porosity remains in the desired gas access area, 205. Theporous PTFE is a convenient substrate for the gas sensor electrode butcan be difficult for printing. The substrate layer 200 has an accessport 205 for entry of a gas sample to be measured, and an optionalfilter 215 covering the access port 205 can be made of virtually anyshape and size to enhance sensor operation in applications. Alsoillustrated in FIG. 1 is an optional backplate 220. The backplate maycomprise a TFE substrate, or other plastic and can be utilized to blockgas access to the entire substrate 200 except for an openingcorresponding to the access port 205. This layer 220 is illustrated notto scale and can be very thin in nature or just fill the pores totallyor partially in layer 200.

As further depicted in FIGS. 2A & 2B, the electrode layer 300 is shownhaving a test electrode 310, reference electrode 320, working (orsensing) electrode 330 and counter electrode 340 formed on one side ofthe porous substrate 200, the side opposite to the gas access andoptional filter. It should be understood that, like other components andfeatures described herein, test electrode 310 and reference electrode320 may be omitted entirely if desired or not needed by the sensingsystem. The substrate layer 200 also depicts the area 210 to beencapsulated by the encapsulation layer 500. The electrolyte layer 400is within the encapsulation layer 500 and adjacent to and inelectrolytic contact with the electrode layer 300.

The test electrode 310 may allow the printed gas sensor to self-test fordegradation or failure of the working electrode 330 (and therefore, theability of the printed gas sensor 100 to accurately sense and measure atarget gas concentration) and be aware of sensor condition. The testelectrode 310 may be arranged to sense a gas normally present in theatmosphere, e.g., oxygen, to detect the decomposition of the electrodesor electrolyte or provide added temperature compensation capability orsimilar performance enhancements over time. The test electrode 310 mayalso be arranged to generate a test gas within the sensor and in acontrolled amount to effect a test of the operation of the sensor (i.e.,working electrode 330). The test electrode 310 may comprise a catalystcomprising Pt, Pd, Au, Ag, Ru, ruthenium dioxide, Cr, Mn, Fe, Co,carbon, or combinations or alloys or composites suitable to the need andfunction in the sensor. The test electrode 310 can also be electricallypulsed, for example during gas generation, electronic sensor testing, orthe like.

In this example, the working electrode 330 has a sensing area 335 and iscomprised of a platinum metal catalyst composite for CO sensing, thoughother metal catalysts may be used as disclosed above for CO or for othergas sensors. The reference electrode 320 may be comprised of Ptcomposite, Ag/AgCl, Pb/PbO2, or other material suitable to the sensordesign. In this particular example for a CO sensor, Pt-black compositeis used. The counter electrode 340 may be comprised of Pt, Au, Pb/PbO2,or any suitable material for the particular sensor. In this particularexample, a CO sensor with Pt-Black and Teflon particle composite isused.

The electrolyte layer 400 is in electrolytic contact with the electrodelayer 300 and comprises lithium perchlorate in propylene carbonate oralternatively polyvinyl alcohol with sulfuric acid or sulfuric acid in aporous glass or porous plastic matrix. The electrolyte can use ionicliquids or solid polymers or other electrolytes known in the sensorsystem arts, such as, for example fuel cells and batteries. Theelectrolyte layer 400 covers the sensing area 335 of the workingelectrode 330 and extends to the other sensor electrodes such that allelectrodes in the sensor are in electrolytic contact (i.e. in contactthrough the electrolyte). In some cases, it may also be desirable forthe thin film electrolyte to be in contact with an electrolyte reservoir410 formed on or over the electrode layer 300 and it may be formed in asection of the encapsulation layer. The electrolyte reservoir 410 mayinclude an expansion area 410A to house electrolyte that expands fromthe electrolyte reservoir 410 when electrolyte reservoir 410 is full,such as when the volume of the electrolyte expands in a high relativehumidity environment.

The encapsulation layer 500, as shown in FIG. 1, encapsulates theelectrode 300 and electrolyte layer 400 thereby forming an integratedstructure with the porous substrate 200. As further depicted in FIG. 3A,encapsulation layer 500 comprises a capillary channel layer 510 having acapillary channel 520 for entry of the electrolyte layer. Capillarychannel layer 510 also defines a bucket volume 515 where electrolyte canreside during expansion from accumulated moisture from the air. Thecapillary channel layer 510 is an optional component to the operation ofthe sensor The encapsulation layer 500 may further comprise a cap layer530 having a gas vent hole 540, as depicted in FIG. 3B. The gas vent isuseful for filling the electrolyte in one exemplary embodiment and forapplications wherein the sensor is used and the pressure is changed ofthe sensed ambient at a certain rate. The capillary channel layer 510may further include one or more capillary channel access ports 510A thatallow electrolyte to flow through the capillary channel layer 510. Thecapillary channel layer 510 and cap layer 530 shown were laser cut frompolypropylene be formed using plastic film stamping operations in thisexample, laser cutting or die cutting to create contours and/or holes.The two layers are packaged together by heat sealing them together oralternatively by adhesive bonding, and then the two layer package isheat sealed or otherwise bonded to the porous substrate layer 200. It isdesired that the electrode or a conductor emerge through theencapsulation so that external contact to the electrodes encapsulatedwithin is made. The connection means can be by conductor wire,conductive plastic, printed runner, pre-metalized plastic, or anymaterial that is a conductor and can be sealed such that the electrodeand electrolyte materials cannot escape from the sensor over many yearsof use. The cap layer 530 also depicts the area 535 to be encapsulatedand sealed, using, for example, a heat seal or a PSA. While theencapsulation layer is shown having two sub-layers, it should beunderstood that the encapsulation layer may comprise one layer having acapillary channel, bucket volume, and gas vent or any other optionalfeature formed therein. Pre-forming these features should save assemblytime and cost in manufacture.

FIGS. 4-7 depict another example of a printed gas sensor operable todetect and measure carbon monoxide levels. Referring to FIG. 4, anexemplary version of a printed gas sensor 1000 is depicted. The printedgas sensor 1000 shown may measure carbon monoxide levels; however, othertarget gases may be used. Sensor 1000 comprises: a porous substrate1100; an electrode layer 1200; a wicking layer 1300, a liquid or gelelectrolyte layer 1400 in contact and permeating the wick 1300 and inelectrolytic contact with the electrode layer 1200; and an encapsulationlayer 1500 that encapsulates the electrode layer 1200, wicking layer1300, and electrolyte layer 1400 thereby forming an integrated structurewith the porous substrate 1100.

As further depicted in FIG. 5, the substrate layer 1100 comprises porousPTFE, though other porous materials may be used, and has a gas accesshole 1105 where the porosity of the PTFE is maintained and everywhereelse can be closed off either by a substrate plastic bonded to it or byadditives painted on it. The substrate layer 1100 further comprises afilter 1115, depicted in FIG. 4, covering the gas access hole which canbe a plastic part used to close off the porous PTFE from the ambienteverywhere except the gas access part. Note that the gas access part ispreferred to align with the working electrode printed on the oppositeside of the porous PTFE. The substrate 1100 is further shown in FIG. 6having four electrodes printed onto it. The electrode layer 1200 isshown having a test electrode 1210, reference electrode 1220, working(or sensing) electrode 1230 and counter electrode 1240 formed on oneside of said porous substrate 1100. As mentioned above, test electrode1210 and reference electrode 1220 may be omitted entirely if desired.The substrate layer 1100 also depicts the area 1110 to be encapsulatedby the encapsulation layer 1500.

In this example, the working electrode 1230 has a sensing area 1235, andis comprised of a platinum metal catalyst, though other metal catalystsmay be used as disclosed above. The counter electrode 1240 may becomprised of Pt-black-PTFE porous electrode composite in this particularexample also for a CO sensor to result.

The wicking layer 1300 of exemplary printed sensor 1000 is shown in FIG.4. The wicking layer 1300 may be screen printed or inkjet printed ontothe electrode layer 1200. The wicking layer 1300 may comprise silicates,Nafion, silicon carbide, carbon, graphite, glass fiber filter paper,porous polypropylene, and Teflon. The wicking layer 1300 may alsocomprise other battery separator type materials that will wick and holdelectrolyte against the electrodes in the sensor; such materials aretypically chosen to be compatible with the electrolyte. In thisembodiment, the wick may not be electrically conductive and may bewettable by the chosen electrolyte.

As depicted in FIG. 4, the electrolyte layer 1400 is in electrolyticcontact with the electrode layer 1200 and comprises lithium perchloratein propylene carbonate or alternatively polyvinyl alcohol with sulfuricacid, polyethylene oxide and lithium chloride, ionic liquids, aceticacid with certain salts or KCl or sodium, potassium or cesium hydroxidesolutions. The electrolyte layer 1400 covers the sensing area 1235 ofthe working electrode 1230. That is, it provides electrolytic contactwith the electrodes in the electrode layer 1200, and generally via anelectrolyte reservoir 1410 which is an empty, unfilled portion of theelectrolyte layer formed within the encapsulation area 1500. In onefurther embodiment, an electrode contact area 1250 is outside theencapsulation area 1500 wherein electronic contact is made to theelectrodes.

The encapsulation layer 1500, as shown in FIG. 4, encapsulates theelectrode layer 1200, wicking layer 1300, and electrolyte layer 1400thereby forming an integrated structure with the porous substrate 1100through which no materials can pass over long times. The encapsulationlayer 1500, as further depicted in FIG. 7, comprises a capillary channel1510, a bucket volume 1520 and a gas vent hole 1530. In this embodiment,the capillary channel layer 1510 and bucket volume 1520 shown are formedfrom polypropylene using a plastic film stamping operation to create thecontours. The gas vent hole is laser cut. The encapsulation layer 1500is heat sealed to the porous substrate layer 1100. These features allowfor convenient filling with electrolyte solution but are not required ifthe wicking layer already contains the electrolyte or the electrolyte isplaced over the electrodes before encapsulation steps are taken.

Referring to FIG. 8, the electrochemical reaction zone for a gaseousanalyte that has been created in the exemplary sensors of FIGS. 1-3 and4-7 is depicted. The carbon monoxide electrochemical reaction takesplace in the working electrode at the triple phase boundary 840 of themetal catalyst (surface of the working electrode), the electrolyte(touching the surface of the working electrode), and the gas alsoarriving at and touching the surface of the electrode and electrolyteand perhaps being considered dissolved in the electrolyte in the triplephase boundary region. As shown in FIG. 8, the carbon monoxide gas 810enters through the gas access hole and through the pores of a porousPTFE substrate 820 and contacts the catalyst particles 825 that are atleast partially wetted with electrolyte 830. It should be understoodthat FIG. 8 is a schematic depiction of the electrochemical reactionzone for a gaseous analyte and is not to scale. As such, in someembodiments, the catalyst particles 825 are in physical contact suchthat electricity can be conducted through the catalyst particles 825.

Referring now to FIGS. 9 and 10, an exemplary version of a printed gassensor 600 is shown. The printed gas sensor 600 comprises a sensor layer630 coupled to a filter assembly 610 on a first side of the sensor layer630 and a reservoir assembly 620 on a second, opposite side. The sensorlayer 630 comprises a substrate layer 640 coupled to an encapsulationlayer 680 using a PSA 650. This forms an integrated structure anddefines an electrolyte cavity 675 located between the substrate layer640 and an encapsulation layer 680. One or more electrodes 673 arecoupled to a wick 670 (FIG. 10) positioned within the electrolyte cavity675. The substrate layer 640 can be coupled to the filter assembly 610and the encapsulation layer 680 can be coupled to the reservoir assembly620.

In some embodiments, the printed gas sensor 600 includes a filterassembly 610 comprising a fill port layer 613 and a filter cavity ring614. The fill port layer 613 comprises formed plastic or other suitablematerials, such as, for example, PET and includes one or more filterholes 612 for gas entry into the filter assembly 610. The filter cavityring 614 comprises carbon or other suitable materials, such as, forexample a combination of 300 LSE and PET. The filter cavity ring 614 iscoupled to the fill port layer 613 using PSA 650 on a first side of thefilter cavity ring 614 and coupled to the substrate layer 640 using PSA650 on a second, opposite side of the filter cavity ring 614. The PSAcreates a hermetic seal, preventing gas entry into the sensor layer 630or the filter assembly 610 at any location except the one or more filterholes 612.

When the fill port layer 613 is coupled to the filter cavity ring 614 acavity is formed below the fill port layer 613 and within the filtercavity ring 614. Filter material 611 can be located within this cavity.The filter material 611 may permit access of certain target gases, suchas, for example, CO and prevent access of certain gases, such as, forexample H₂S, HO₂, SO₂, O₂, NO₂, HCl. The filter material 611 maycomprise a C/KMnO₄ plug chemically configured to remove H₂S, HO₂, SO₂,O₂, NO₂, HCl, condensable hydrocarbons, alcohols, or the like. TheC/KMnO₄ plug may include carbon cloth, activated carbon, or other filtermaterials impregnated with KMnO₄ or other reactive materials. Thereactive materials can be designed to prevent entry of interfering gaseswhile allowing entry of target gases, or analytes. Alternately, thefilter material 611 may comprise a carbon and KMnO₄ on alumina mixture,or other carbon mixtures. In operation, gases can enter the filter holes612 at the top of the fill port layer 613 and reach the filter material611. Any gas that is able to travel through the filter material 611(e.g., CO) can enter the sensor layer 630 of the printed gas sensor 600.In some embodiments, the printed gas sensor 600 does not include afilter assembly 610. In these embodiments, gases can enter directly intothe sensor layer 630.

Still referring to FIG. 9, the printed gas sensor 600 may include areservoir assembly 620 coupled to the sensor layer 630 opposite thefilter assembly 610. The reservoir assembly 620 can function as anoverflow chamber for electrolyte 674 disposed within the electrolytecavity 675 of the sensor layer 630. Environmental factors, such asrelative humidity, may cause the electrolyte 674 and other liquids toexpand within the electrolyte cavity 675. The reservoir assembly 620allows some of the expanded electrolyte 674 and other expanded fluidswithin the printed gas sensor 600 to flow into the reservoir assembly620 without breaking the hermetic seals located throughout the printedgas sensor 600. To accommodate electrolyte 674 expansion, the volume ofthe reservoir assembly 620 can be between three and six times largerthan the volume of the electrolyte layer 675, for example, when theelectrolyte 674 comprises 4M H₂SO₄. This fill volume allows the printedgas sensor 600 to accommodate fluid expansion that can occur when theprinted gas sensor 600 is used in high relative humidity conditions,such as a 95% relative humidity environment.

The reservoir assembly 620 may comprise a reservoir fill port layer 622having one or more reservoir overflow holes 623 and a reservoir cavityring 621. The reservoir fill port layer 622 comprises formed plastic orother suitable materials, such as, for example, PET. The reservoircavity ring 621 comprises formed plastic or other suitable materials,such as, for example a combination of 300 LSE and PET. The reservoircavity ring 621 is coupled to the reservoir fill port layer 622 on afirst side of the reservoir cavity ring 621 and coupled to a sensorlayer 630 on an opposite side of the reservoir cavity ring 621. The oneor more reservoir overflow holes 623 in the reservoir fill port layer622 allow an electrolyte 674 to access the electrolyte cavity 675 (FIG.10) of the sensor layer 630 through the reservoir assembly 620. Onceelectrolyte 674 enters electrolyte cavity 675, the reservoir overflowholes 623 can be hermetically sealed or plugged, preventing liquids orgases from entering the sensor layer 630 except through gas accessregions 643 (FIG. 10). In some embodiments, the reservoir assembly 620further comprises a reservoir plug 624 coupled to the reservoir fillport layer 622, opposite the reservoir cavity ring 621. The reservoirplug 624 can hermetically seal the reservoir overflow holes 623. In someembodiments, the reservoir plug 624 is coupled to the reservoir fillport layer 622 after the electrolyte 674 has filled the electrolytecavity 675. In some embodiments, the reservoir cavity ring 621 and thereservoir fill port layer 622 are integral forming an integral overflowchamber or “bucket”.

Referring now to FIG. 10, sensor layer 630 of the printed gas sensor 600of FIG. 9 is depicted in more detail. The sensor layer 630 comprises asubstrate layer 640 having a first partially porous substrate 641 and asecond partially porous substrate 642, coupled together using PSA 650disposed on substantially the entire surface of the first partiallyporous substrate 641 facing the second partially porous substrate 642.The first partially porous substrate 641 comprises polycarbonate and thesecond partially porous substrate 642 comprises LSE and PET. In otherembodiments, the first partially porous substrate 641 and the secondpartially porous substrate 642 may comprise other formed plastics, suchas PTFE. The first partially porous substrate 641 comprises gas accessregions 643 that allows gas, for example a target gas, to enter thesensor layer 630. Gas access regions 643 may be holes. Gas accessregions 643 may also be porous regions in the first partially poroussubstrates 641 formed by selectively coating the first partially poroussubstrate 641 with polyimide. Regions of the first partially poroussubstrate 641 covered with polyimide block gas access and regions offirst partially porous substrate 641 without polyimide form gas accessregions 643 to allow gases including one or more target gases to enterthe sensor layer 630. The second partially porous substrate 642 maycomprise a slot 644 configured such that a PTFE disk 645 can be disposedwithin the slot 644. The slot 644 is located substantially coaxial withthe gas access regions 643 and may have a diameter larger than thediameter of the gas access regions 643 such that when a PTFE disk 645 ispositioned within the slot 644 it couples to a portion of the firstpartially porous substrate 641 overhanging the slot 644 and issubstantially coaxial with the gas access regions 643. The PTFE disk 645may comprise a partially porous PTFE membrane such as, for example,MuPore. Gas that enters the sensor layer 630 through the gas accessregions 643 diffuses through the PTFE disk 645 and reaches theelectrodes 673 positioned within the electrolyte cavity 675. It shouldbe understood that the substrate layer 640 may alternatively comprise asingle partially porous substrate including one or more gas accessregions and further including a slot for housing a PTFE disk.

The substrate layer 640 can further comprise a printed runner 646printed directly onto the second partially porous substrate 642. Theprinted runner 646 faces the electrolyte cavity 675 and comprises anon-porous hydrophobic material that does not soak up any electrolyte674. The printed runner 646 can be stamped, vapor deposited, or thelike, onto the second partially porous substrate 642. The printed runner646 is patterned, allowing it to engage with a variety of electrodes 673within the electrolyte cavity 675. In some embodiments, the printedrunner 646 comprises one or more conductive traces having conductiveink.

Still referring to FIG. 10, an encapsulation layer 680 comprises anelectrolyte fill port layer 681 with one or more electrolyte accessholes 682 coupled to an encapsulation cavity ring 683. The electrolytefill port layer 681 and the encapsulation cavity ring 683 compriseformed plastic, such as PET or a combination of LSE and PET. Theencapsulation cavity ring 683 is further coupled to the second partiallyporous substrate 642 opposite the electrolyte fill port layer 681. PSA650 is disposed between the electrolyte fill port layer 681, theencapsulation cavity ring 683 and the second partially porous substrate642 of the substrate layer 640, hermetically sealing the encapsulationlayer 680 and forming an electrolyte cavity 675 within the encapsulationcavity ring 683. The electrolyte cavity 675 can house an electrolyte 674and a wick 670 coupled to one or more electrodes 673. In someembodiments, the electrolyte fill port layer 681 and the encapsulationcavity ring 683 form a single integral, “bucket” structure which iscoupled to the second partially porous substrate 642 and houses theelectrolyte cavity 675. The electrolyte access holes 682 in theelectrolyte fill port layer 681 allow electrolyte 674 to enter theelectrolyte cavity 675.

The sensor layer 630 depicted in FIG. 10 further comprises one or moreelectrodes 673 coupled to a wick 670. The wick 670 can comprise porousglass fiber or glass fiber filter paper. In this embodiment, the one ormore electrodes 673 are screen printed, inkjet printed, stamped, orstenciled onto the wick 670. Electrodes 673 can be printed and curedonto the wick 670 before the wick 670 is assembled into the printed gassensor 600. For example, the wick 670 can be embedded with electrode 673creating a wick-electrode assembly having three regions. A first regionincludes only wick 670, a second region includes electrode 673 embeddedin the wick 670, and a third region includes only electrode 673,extending above the wick 670. The second region is graded such that itcontains gradually more electrode 673 when measured from a boundary withthe first region (comprising only wick 670) to the boundary with thethird region (comprising only electrode 673). This transition from wick670 to electrode 673 facilitates the intimate contact and transition ofelectrolyte 674 through the wick 670 to the electrode 673, encouragingwetting. Further, the wick-electrode assembly creates a strongstructural bond between the electrode 673 and the wick 670.

The glass fiber or a glass fiber filter paper of the wick 670 cantolerate higher temperatures than the materials of the substrate layer640 or the encapsulation layer 680. For example, the wick 670 cantolerate a curing temperature of about 300° C. This allows the one ormore electrodes 673 to be cured onto the wick 670 before being assembledinto the printed gas sensor 600. The wick 670 is also compressible. Insome embodiments, the height of the electrolyte cavity 675 is less thanthe initial height of the wick 670. When the wick 670 is assembledwithin the electrolyte cavity 675, the wick 670 is compressed to fitwithin the electrolyte cavity 675.

The one or more electrodes 673 may comprise PTFE ink mixed with metalcatalyst and additives to form a PTFE ink black composite electrode. Inparticular, the one or more electrodes 673 may comprise 1.44-1.45 g Pt,0.16±0.1 g graphite carbon and 0.80-0.81 g Teflon suspension (0.48 gPTFE). The Teflon suspension can be a mixture of TFE particles less than1 m diameter, water, surfactant, and 3 mL ethyl cellulose solution. Insome embodiments, the electrodes 673 are printed from inks that areprepared from dry PTFE particles instead of an aqueous PTFE solution. Inother embodiments, the electrodes 673 are printed from inks that areprepared from polypropylene powder or polyethylene powder, as explainedin more detail below.

Referring still to FIG. 10, the wick 670 with one or more electrodes 673are positioned within the electrolyte cavity 675. Once the wick 670 isassembled into the electrolyte cavity 675, electrodes 673 that arepositioned on the wick 670 can mate with the printed runner 646. When anelectrolyte 674, such as H₂SO₄, is introduced into the electrolytecavity 675 it contacts the wick 670. The electrolyte 674 can seep intothe wick 670 and contact the electrode 673. Once the electrolyte 674 andelectrode 673 are in contact, and a target gas, such as CO enters theprinted gas sensor 600 through the gas access regions 643, anelectrochemical reaction is generated in the electrolyte cavity 675. Theelectrochemical reaction between the electrode 673, the electrolyte 674,and a target gas generates an electric current in the printed runner 646and sends electric signal to one or more circuits connected to theprinted runner 646 at one or more electrical contact points 661. Thiselectric signal communicates to one or more circuits that a target gasis detected in the printed gas sensor 600.

Referring still to FIG. 10, a printed runner 646 faces the wick 670 andcontacts at least one electrode 673, and extending along a length of theprinted runner 646. Alternative embodiments may include two or moreprinted runners 646. The printed runner 646 terminates at one or moreelectrical contact points 661 coupled to the outer portion of theprinted runner 646. The electrical contact points 661 facilitate anelectrical connection between the printed runner 646 and one or morecircuits, conductive wires, or the like. This allows electrical currentgenerated by an electrochemical reaction between the electrode 673,electrolyte 674, and a target gas to travel along the printed runner646, from the electrode 673 to one or more circuits. The one or morecircuits may be arranged on a printed circuit board. The one or morecircuits can use the electrical current generated by the electrode 673,electrolyte 674, and target gas to trigger a signal or an alarmfunction. In alternative embodiments, the substrate layer 640 may notcomprise a printed runner 646. In this embodiment, a printed runner 646is coupled to the partially porous substrate facing the wick 670. Insome embodiments, printed runner 646 comprises one or more conductivetraces having conductive ink.

Referring now to FIG. 11A, the embodiments of FIGS. 9 and 10 aredepicted in an exploded view. Referring now to FIG. 11B, the filterassembly 610, reservoir assembly 620, and sensor layer 630 of FIGS. 9and 10 are depicted in printed form. While FIGS. 11A and 11B depict theoptional filter assembly 610 and optional reservoir assembly 620, itshould be understood that embodiments of the printed gas sensor 600without a filter assembly 610 or a reservoir assembly 620 arecontemplated.

Referring now to FIG. 12, a sensor layer 730 of an embodiment of aprinted gas sensor 700 comprising one or more electrodes 773 curable attemperatures lower than the melting point and deformation point of thematerials of the sensor layer 730 is depicted. The sensor layer 730comprises a solid substrate 741 coupled to an encapsulation housing 781forming an electrolyte cavity 775 between the solid substrate 741 andthe encapsulation housing 781. The solid substrate 741 can be partiallyporous to allow gas to enter the printed gas sensor 700. Gas accessregions 743 can be formed by selectively coating the solid substrate 741with polyimide. Regions of the solid substrate 741 covered withpolyimide block gas access and regions of the solid substrate 741without polyimide form gas access regions 743 to allow gases includingone or more target gases to enter the sensor layer 730. The gas accessregions 743 can also be one or more holes. The solid substrate 741 isalso hydrophobic and oligophobic to prevent electrolyte from absorbinginto the solid substrate 741 and blocking the gas access regions 743.The encapsulation housing 781 includes one or more encapsulation holes782. The solid substrate 741 and the encapsulation housing 781 compriselow temperature plastics such as polycarbonate substrate and PETsubstrate. Polycarbonate substrate and PET substrate are chemicallyinert and have melting points less than or equal to 260° C. Further, oneor more printed runners 760 are coupled to the solid substrate 741 andare configured to carry electric current generated in the electrolytecavity 775 to one or more contact points 761. The one or more contactpoints 761 can be in electrical contact with one or more circuits, suchas, for example, a printed circuit board.

In the embodiment depicted in FIG. 12, one or more electrodes 773 areprinted directly on the solid substrate 741. The electrodes 773 are madefrom ink compositions which cure at temperatures below the melting pointor deformation point of both the solid substrate 741 and theencapsulation housing 781. This allows the entire sensor layer 730 toundergo the curing process for the electrode 773 without melting thesolid substrate 741 or the encapsulation housing 781. One exemplary lowtemperature electrode ink composition is a polypropylene mixturecomprising polypropylene, catalyst, solvent, and additives, such as, forexample, platinum, palladium, or alloys or supported catalysts likeplatinum on carbon. Another exemplary electrode ink compositioncomprises polyethelene, catalyst, solvents, and additives, such as, forexample, platinum, palladium, or alloys or supported catalysts likeplatinum on carbon. These electrode ink compositions cure attemperatures less than or equal to 250° C. and can form porous gasdiffusion electrodes that are partially hydrophobic. Further, electrodes773 printed from these electrode ink compositions form a triple phaseboundary with an electrolyte 774 and a target gas. When an electrolyte774 contacts an electrode 773 a contact angle of 70° or greater isformed, partially wetting the electrode 773. For example, an electrolyte774 comprising H₂SO₄ may have a contact angle of 75° or greater. Theprinted gas sensor 700 depicted in FIG. 12 may further comprise a filterassembly and a reservoir assembly coupled to the sensor layer atopposite sides of the sensor layer. The filter assembly and thereservoir assembly may be the same or substantially similar to theembodiments depicted in FIG. 9.

Referring now to FIG. 13, a sensor layer 930 of an exemplary printed gassensor 900 is shown. Sensor layer 930 comprises a high temperature uppersubstrate 941 partially coated with polyimide to form one or more porousgas access regions 943. Regions of the high temperature upper substrate941 covered with polyimide block gas access and regions of the hightemperature upper substrate 941 without polyimide form gas accessregions 943 to allow gases including one or more target gases to enterthe sensor layer 930. In some embodiments, gas access regions 943 maycomprise one or more holes. The high temperature upper substrate 941 iscoupled to a high temperature lower substrate 942 using one or moresealer spacers 990, forming an electrolyte cavity 975 between the hightemperature upper substrate 941 and the high temperature lower substrate942. The sealer spacers 990 are a solid material that can be used toweld the high temperature upper substrate 941 to the high temperaturelower substrate 942 using a thermoplastic welding rod or other exemplaryplastic welding method. During this welding process, the material of thesealer spacer 990 is selectively melted and can seep into the pores ofthe high temperature upper substrate 941 and the high temperature lowersubstrate 942; gluing, welding, or otherwise fastening the hightemperature upper substrate 941 and the high temperature lower substrate942 together. This forms a hermetic seal and creates the electrolytecavity 975 between the high temperature upper substrate 941 and the hightemperature lower substrate 942. In some embodiments, the hightemperature upper substrate 941, the high temperature lower substrate942, or both can be partially porous. This partial porosity can beformed by pattering each substrate 941, 942 with a photoresist materialusing a UV masking process.

The high temperature upper substrate 941 and the high temperature lowersubstrate 942 may comprise PTFE, polyimide, or any other hightemperature, chemically inert material and the sealer spacers 990 mayinclude FEP or other chemically inert, flowable bonding material thatcan withstand corrosive chemicals used in electrolytes, electrodes, andanalytes and can withstand temperatures of 260° C. or higher. Thematerials of the high temperature upper substrate 971, high temperaturelower substrate 972, and sealer spacers 990 can withstand curingtemperatures of at least 260° C. For example, polyimide can withstandtemperature of up to about 400° C. and PTFE can withstand temperaturesof up to about 330° C. These high temperature materials allow the sensorlayer 930 to be assembled onto a circuit board during circuit boardproduction and placed in a solder reflow furnace at 260° C. for variousperiods of time, for example, nine seconds.

One or more electrodes 973 are coupled to the high temperature uppersubstrate 941 using inkjet or screen printing techniques and arepositioned within the electrolyte cavity 975. The one or more electrodes973 may comprise TFE bonded electrodes curable at temperatures fromabout 260° C. to about 330° C., such as, for example, 300° C. Theelectrodes may comprise PTFE ink, a metal catalyst, solvents, andadditives. Some electrodes 973 may be configured to interact with theelectrolyte 974 and a target gas to create an electrochemical reactionand generate an electrical current. Other electrodes operate as a bunnytester or a conductivity tester. An electrode 973 operating as aconductivity tester may be configured to continuously or intermittentlycheck whether the cell is functioning properly and may measure andmonitor the concentration of the electrolyte 974. In some embodiments,multiple electrodes 973 may be configured to each detect differenttarget gases. For example, a first electrode 973 can detect CO and asecond electrode (not shown) can detect gases such as H₂S, O₃, SO₄, orNO₂. The electrolyte cavity 975 can be partially filled with a wick thatcontacts each electrode 973 and wets each electrode 973 with electrolyte974. Further, in embodiments in which electrolyte 974 comprises H₂SO₄,the volume of the electrolyte 974 can expand or contract depending onthe relative humidity of the environment surrounding the printed gassensor 900. The volume of a fixed amount of electrolyte 974 can expandthree to six times in volume in a high relative humidity environment. Asdescribed in more detail above, the reservoir assembly coupled to thesensor layer 930 can be configured to house overflow electrolyte.

Still referring to FIG. 13, the hermetically sealed electrolyte cavity975 houses the one or more electrodes 973 and the electrolyte 974,creating a location for electrochemical reactions between the electrode973, the electrolyte 974, and the target gas. The materials of the hightemperature upper substrate 941, high temperature lower substrate 942,and sealer spacers 990, such as, for example, PTFE, can be chemicallyinert; allowing any electrolyte 974 to enter the electrolyte cavity 975without generating a chemical reaction between the materials of the hightemperature upper substrate 941, the high temperature lower substrate942, and the electrolyte 974. Exemplary electrolytes 974 includecorrosive acids, bases, and salts, such as, for example, H₂SO₄, whichcan withstand curing temperatures up to 340° C. Other exemplaryelectrolytes 974 include solvents and RTILs, such as, for example, ionicliquids which can withstand curing temperatures up to 360° C.

Still referring to FIG. 13, printed runners 960 are coupled to the hightemperature upper substrate 941, facing the electrolyte cavity 975. Theprinted runners 960 are configured to carry electric current generatedin the electrolyte cavity 975 to one or more contact points 961. The oneor more contact points 961 can be in electrical contact with one or morecircuits, such as, for example, a printed circuit board. The printedrunners 960 can be compositionally and functionally the same as thosedescribed above with respect to FIGS. 10 and 12. The printed gas sensor900 may further comprise a filter assembly and a reservoir assemblycoupled to the sensor layer at opposite sides of the sensor layer 930.The filter assembly and the reservoir assembly may be the same orsubstantially similar to the embodiments depicted in FIG. 9.

The diameter of a gas inlet hole of any of the embodiments describedabove may be varied to alter the sensitivity range of target gasdetection in the sensor layer. In each embodiment, the amount of gasentering the sensor should match the reactivity of the sensor. As thegas inlet hole diameter increases, more catalyst should be present inthe electrode to optimize the signal to noise ratio of the printed gassensor. For example, a gas inlet hole with a diameter of 1 mm can bematched with a 5-10 mg WE catalyst and a 0.25 mm gas inlet hole can bematched with a 2.5 mg catalyst. The specific sensitivity and signal tonoise ratio can be selected based on the detection range and outputrequirements of the printed gas sensor. For example, a high target gasdetection range is between about 0 ppm and about 1000 ppm and a lowtarget gas detection range is between about 0 ppm and about 10 ppm. Aprinted gas sensor with the low target gas detection range will haveless catalyst and a smaller gas inlet hole than a printed gas sensorwith the high target gas detection range. Target matching creates scaledsensitivity levels in the printed gas sensors. For example, a printedgas sensor with a low target gas detection range can detect low levelsof target gas in part because it can minimize background noise. Also, aprinted gas sensor with a high target gas detection range can detecthigh levels of target gas while tolerating high levels of backgroundnoise.

In the embodiments described herein, the electrolyte acid concentrationcan be selected to match the relative humidity of the environment inwhich the printed gas sensor will be used. For example, if the relativehumidity of the environment is greater than 95% continuous (e.g., coalmines) then about a IM acid electrolyte should be used. If relativehumidity of the environment is less than 10% (e.g., dry city gas), thenabout a 10M acid electrolyte should be used. In some embodiments, asingle sensor design can operate in a relative humidity environmentranging from 5-95% relative humidity. In these embodiments, the printedgas sensor can further include the reservoir assembly coupled to thesensor layer of the printed gas sensor. An environment with variablerelative humidity can cause the electrolyte to expand and contract. Byincluding the reservoir assembly, the electrolyte can expand andcontract into and out of the reservoir assembly without breaking thehermetic seal of the sensor layer. In some embodiments, the reservoirassembly should have a volume multiple times larger than the electrolytecavity. For example, when the electrolyte is H₂SO₄, the volume of thereservoir assembly can be three to six times greater than the volume ofthe electrolyte cavity.

In some embodiments, porous electrodes comprise alternative polymercomponents. In some embodiments, the electrodes comprise dry PTFEparticles which replace the standard PTFE aqueous dispersion particles.In some embodiments, the electrodes are prepared with polypropyleneparticles replacing the standard PTFE aqueous dispersion particles. Inother embodiments, polyethylene particles replace the standard PTFEparticles. One advantage of replacing the PTFE with the polymers notedabove is the electrodes will cure at lower temperatures. This allowsdirect printing of the electrodes onto the plastic substrates and parts.The electrodes can be cured onto the plastic substrates and parts duringthe assembly process. Special handling or pick and place operations arenot required. This can reduce and simplify the processing steps.

In any of the embodiments described above, the manufacturing process ofthe printed gas sensors can be scalable, for example, in bulk sheetscomprising 10-100 printed gas sensors per sheet, such as, for example, asheet with 60 printed gas sensors. In some embodiments, the scalablemanufacturing process can produce the printed gas sensors of the variousembodiments in one or more sheets that can be a variety of sizes. Forexample, printed as sensors can be manufactured in 2×5 sheets comprising10 printed gas sensors, 5×5 sheets comprising 25 printed gas sensors,θ×10 sheets comprising 100 printed gas sensors, or any other size andshape of scalable, manufactured sheet. It should be understood that theprocess of manufacturing printed gas sensors can be scaled to any sizedesired, such as sheets having 500 printed gas sensors, 1000 printed gassensors, or more.

In some embodiments, the electrodes are prepared with an ethyl cellulosesolution. In this embodiment, 5 g 10 cP ethylcellulose is dissolved intoa 75 mL isophorone 25 mL diacetone alcohol solution. The solution isdissolved by stirring in a covered container overnight at 50° C. to 75°C. In some embodiments, the 75 mL isophorone is replaced with 75 mLoctanol which reduces the toxicity and the odor. Other solvents such asnonanol or decanol can also be used to replace isophorone in the ethylcellulose solution. The solvent must have at least the minimum polarityrequired to dissolve the diacetone. In a preferred embodiment, thesolvent evaporates at a slow pace. In some embodiments, ethyl cellulosecan be added to adjust the viscosity for printing. In a preferredembodiment, the dispersion is stable with a low evaporation rate andgood viscosity. When the electrode is cured (typically between 60° C.and 250° C., at a temperature that does not damage the plasticsubstrate) the solvent must leave the composite electrode behind.

In some embodiments, the electrodes comprise a catalyst ink suspensionof 23-25% weight PTFE. More specifically, the catalyst ink suspensioncomprises a mixture of 1.44-1.45 g Pt, 0.16±0.01 g graphite carbon and a0.80-0.81 g PTFE suspension comprising 0.48 g PTFE particles less than 1μm, water, surfactant, and 3 mL ethyl cellulose solution or octanolsolution.

In some embodiments, inks are prepared from dry PTFE particles insteadof a standard PTFE aqueous solution. Dry PTFE particles are preparedfrom dry PTFE powder and are suspended directly into the solvent. Theyare not placed in an aqueous suspension like the standard PTFEsuspension. Electrodes prepared from dry PTFE particles yield 20-45% ofthe response of electrodes prepared using a PTFE aqueous solution. FIG.14 illustrates the response signal in μAmps of electrodes comprising 23%and 30% dry PTFE particles and electrodes comprising a PTFE aqueoussolution. The signal peaks are measuring, from left to right in FIG. 14,the addition of 50 ppm target gas, 100 ppm target gas, and 400 ppmtarget gas, respectively.

In some embodiments, electrodes are prepared using low temperaturebinders such as polypropylene and polyethylene powders. These powdersare hydrophobic binders. The densities of polypropylene (PPRO) andpolyethylene (PE) are lower than the density of PTFE. (PTFE=2.0-2.2g/cc; PPRO=0.93 g/cc; PE=0.97 g/cc). Because of the lower density, theoptimized weight percentage for PPRO and PE is lower than PTFE (15-17wgt % PPRO, PE vs. 23-25 wgt % PTFE).

In some embodiments, the PPRO catalyst ink suspension comprises amixture of 1.44-1.45 g Pt (74-78 wgt %), 0.16±0.01 g graphite carbon(8-9 wgt %), 0.25-0.30 g PPRO powder (14-17 wgt %) and a 3 mL ethylcellulose solution. The response of electrodes prepared with 5-7 μm PPROparticles is about 10-20% of the response of electrodes prepared withPTFE aqueous solution. In some embodiments, the PE catalyst inksuspension comprises a mixture of 1.44-1.45 g Pt (74-78 wgt %),0.16±0.01 g graphite carbon (8-9 wgt %), 0.25-0.30 g PE powder (14-17wgt %) and a 3 mL ethyl cellulose solution. The response of electrodesprepared with 2-4 μm PE particles is about 25-30% of the response ofelectrodes prepared PTFE aqueous solution.

In some embodiments, electrode sensitivity can be optimized by reducingthe catalyst particle diameter. FIG. 17 illustrates the effect ofpolymer particle diameter on the sensitivity of electrodes comprising17-23 wgt % polymer and 77-83 wgt % Pt, measuring signal per ppm(μA/ppm) over nominal polymer particle diameter (μm). CO sensitivity isindependent of the specific polymer, provided the requirements for aproper triple phase boundary are met, but depends on the size of theparticles. For example, smaller particles increase CO sensitivity. Insome embodiments, sensor-to-sensor performance can be optimized byreducing the gas inlet hole diameter. In one example, by reducing thegas inlet diameter from 1.5 mm to 1.0 mm, the sensor-to-sensor variationwas improved, as illustrated in FIG. 18. In some embodiments, areduction in gas port area from 3 mm² to 1 mm² reduces the signal by afactor of around 2.5-3.0 (from 50 nA/ppm to 20 nA/ppm) and also reducesthe amount of background noise that can enter the sensor. Both signaland noise can decrease linearly with catalyst area. As the perimeter toarea ratio of the gas port area is increased, there is an improvement inthe signal to noise ratio. The sensitivity of a printed gas sensor canbe optimized by matching the gas inlet hole diameter with the size ofthe electrode layer. For example, a printed gas sensor with a gas inlethole having a 0.5 mm diameter can be optimized with a 5-7 mil thickelectrode layer having a 3 mm diameter and a 2.5 mg/cm² catalyst.

While several devices and components thereof have been discussed indetail above, it should be understood that the components, features,configurations, and methods of integrating and using the devicesdiscussed are not limited to the unique contexts provided above. Inparticular, components, features, configurations, and methods of usedescribed in the context of one of the devices may be incorporated intoor out of any of the other devices. Furthermore, not limited to thefurther description provided below, additional and alternative suitablecomponents, features, configurations, and methods of fabricating andusing the devices, as well as various ways in which the teachings hereinmay be combined and interchanged, will be apparent to those of ordinaryskill in the art in view of the teachings herein.

Having shown and described various versions in the present disclosure,further adaptations of the methods and systems described herein may beaccomplished by appropriate modifications by one of ordinary skill inthe art without departing from the scope of the present invention.Several of such potential modifications have been mentioned, and otherswill be apparent to those skilled in the art. For instance, theexamples, versions, geometrics, materials, dimensions, ratios, steps,and the like discussed above are illustrative and are not required.Accordingly, the scope of the present invention should be considered interms of the following claims and understood not to be limited to thedetails of structure and operation shown and described in thespecification and drawings.

EXPERIMENTS

In the following experiments, alternative polymer components were usedin the formation of porous gas electrodes. Catalytic ink suspensionscomprising different polymer components were used to create electrodes.These electrodes were tested and measured. In particular, experimentswere preformed in which the standard PTFE aqueous dispersion in thecatalytic ink suspension was replaced with dry PTFE particles,polypropylene particles and polyethylene particles, respectively.Experiments were also performed with the goal of optimizing COsensitivity, optimizing sensor to sensor performance, and improving asensor's signal to noise ratio.

The standard PTFE catalyst ink suspension comprises a mixture of1.44-1.45 g Pt, 0.16±0.01 g graphite carbon and a 0.80-0.81 g PTFEsuspension comprising 0.48 g PTFE particles less than 1 μm, water,surfactant, and 3 mL ethyl cellulose solution or octanol solution. Theethyl cellulose solution is a combination of 5 g 10 cP ethyl cellulosemixed with a solution comprising 75 mL isophorone and 25 mL diacetonealcohol that is stirred in a covered container on a hot-plate overnightat 50-70° C. to dissolve the solution. To form the standard PTFEcatalyst ink suspension, the Pt, graphite carbon, PTFE, ethyl cellulose,water, and surfactant are mixed using sonication, dispersed onto thewick or substrate using a screen printing or stenciling process, thencured. When the standard PTFE catalyst ink suspension is cured, thesolvent will evaporate, leaving the composite electrode behind. Itshould be understood that any of the embodiments of the catalyst inksuspension described herein can be formed into electrodes through theprocess of sonicating the catalyst ink suspension, dispersing thecatalyst ink suspension onto the wick or substrate using a screenprinting or stenciling process, then curing the catalyst ink suspension.

The ethyl cellulose solution can also be formed using octanol, nonanol,or decanol in place of isophorone. Octanol is less toxic and lessodorous than isophorone. The solvent used can have a small amount ofpolarity to dissolve the diacetone and should not evaporate too quickly.The solvent can also take up some of the additive ethyl cellulose toadjust the viscosity for printing. This can result in stable dispersion,low evaporation rate, and good viscosity.

Experiment 1

In experiment 1, the catalyst ink suspension was prepared from dry PTFEpowder instead of the standard PTFE suspension (TFE-30™ PTFE) todetermine the difference in response between electrodes made with dryand aqueous PTFE catalyst inks. The catalyst ink suspension formed withdry PTFE powder comprises a mixture of 1.44-1.45 g Pt (67-71 wgt %),0.16±0.01 g graphite carbon (7-8 wgt %), and 0.48-0.49 g PTFE powder(22-25 wgt %) combined with 3 mL ethyl cellulose solution. The particlediameter of dry PTFE powder is about 1-5 μm. This is larger than theparticle diameter of the standard PTFE suspension, which has particlesno larger than 1 μm. FIG. 14 illustrates the response signal in μAmps ofelectrodes comprising 23% and 30% dry PTFE particles and electrodescomprising a PTFE aqueous solution. The signal peaks are measuring, fromleft to right in FIG. 14, the addition of 50 ppm target gas, 100 ppmtarget gas, and 400 ppm target gas, respectively. As illustrated in FIG.14, electrodes formed with a catalyst ink suspension comprising dry PTFEpowder yield 20-45% of the response of electrodes prepared with astandard PTFE suspension. The lower sensitivity of dry PTFE appears tobe related to the particle diameter. This size to sensitivityrelationship is expanded upon in experiment 4, below.

Experiment 2

In experiment 2, the catalyst ink suspension was prepared frompolypropylene (PPRO) powder (a hydrophobic binder) instead of PTFE. PPRObinds at a lower temperature than PTFE. The PPRO powder particles usedwere about 5-7 μm. The optimized weight percentage of PPRO powder islower than PTFE because PPRO has a lower density than PTFE. The densityof PTFE is 2.0-2.2 g/cc and the density of PPRO is 0.93 g/cc. Theoptimized weight percentage of PTFE is 23-25 wgt % and the optimizedweight percentage of PPRO is about 15-17 wgt %. The catalyst inksuspension formed with PPRO powder comprising a mixture of 1.44-1.45 gPt (75-79 wgt %), 0.16±0.01 g graphite carbon (8-9 wgt %), 0.25-0.30 gPPRO powder (14-17 wgt %) combined with a a 3 mL ethyl cellulosesolution.

FIG. 15 illustrates the response signal in μAmps of electrodes formedwith 5-7 μm PPRO powder particles. The signal peaks are measuring, fromleft to right in FIG. 14, with the addition of 250 ppm CO, 500 ppm CO,1000 ppm CO, 2500 ppm CO, 5000 ppm CO and 250 ppm CO, respectively. ThePPRO powder electrode response was about 10-20% of the standard PTFEelectrode response signal. PPRO electrodes responded and recoveredslower than PTFE electrodes.

Experiment 3

In experiment 3, the catalyst ink suspension was prepared frompolyethylene (PE) powder (a hydrophobic binder) instead of PTFE. PEbinds at a lower temperature than PTFE. The PE powder particles usedwere about 2-4 μm. The optimized weight percentage of PE powder is lowerthan the optimized weight percentage of PTFE because PE has a lowerdensity than PTFE. The density of PTFE is 2.0-2.2 g/cc and the densityof PE is 0.97 g/cc. The optimized weight percentage of PTFE is 23-25 wgt% and the optimized weight percentage of PE is about 15-17 wgt %. Thecatalyst ink suspension formed with PE powder comprises a mixture of1.44-1.45 g Pt (75-79 wgt %), 0.16±0.01 g graphite carbon (8-9 wgt %),0.25-0.30 g PE powder (14-17 wgt %) combined with a 3 mL ethyl cellulosesolution.

FIG. 16 illustrates the response signal in μAmps of electrodes formedwith 2-4 μm PE powder particles. The PE powder electrode response wasabout 25-30% of the standard PTFE electrode response. In thisexperiment, PE electrodes responded and recovered slower than PTFEelectrodes. These results indicate a need to optimize the composition toimprove the triple-phase boundary conditions.

Experiment 4

FIG. 17 illustrates the relationship between an electrode's COsensitivity and the particle diameter of the polymers used in theelectrode. It is possible to increase an electrode's CO sensitivity byreducing the particle diameter. The data in FIG. 17 illustrates theeffect of polymer particle diameter on the sensitivity of electrodescomprising 17-23 wgt % polymer and 77-83 wgt % Pt. The CO sensitivity ofthe electrode varies with polymer particle diameter. According to thisexperiment, the sensitivity is independent of the specific polymer. Ifthe requirements for a proper triple-phase boundary are met, polymerparticle diameter becomes an important variable for adjusting COsensitivity.

Experiment 5

FIG. 18 illustrates a relationship between gas port size and sensor tosensor consistency. In experiment 5, a sensor's gas port size wasreduced from a 1.5 mm diameter to a 1.0 mm diameter. This reductionimproved the sensor to sensor consistency to about ±5%.

Experiment 6

In experiment 6, the gas port area of a sensor was reduced from 3 mm² to1 mm², a factor of 9. This reduced the sensor signal from 50 nA/ppm to20 nA/ppm, a factor of about 2.5-3.0. Prior work by Dr. Stetter andcolleagues has shown that sensor baseline currents decrease linearlywith catalyst area and that signal to noise ratio improves as theperimeter to area ratio increases. For example, see Buttner, W. J.,Maclay, G. J., and Stetter, J. R., “Chemical Sensing Apparatus andMethods,” U.S. Pat. No. 5,512,882, issued Apr. 30, 1996; Buttner, W. J.,Maclay, G. J., and Stetter, J. R., “Microfabricated Amperometric GasSensors with an Integrated Design,” Sensors and Materials, 2, 99-106(1990); Buttner, W. J., Maclay, G. J., and Stetter, J. R.,“Microfabricated Amperometric Gas Sensors,” IEEE Trans. On ElectronDevices 35(6), 793 (1988); and Buttner, W. J., Maclay, G. J., andStetter, J. R., “An Integrated Amperometric Microsensor,” Sensors andActuators, B1, 303-307, (1990).

It should be understood that, in general, the smaller sensor enabled bythe design and methods of manufacture described herein result in sensorsscalable to sizes not reached by previous technologies, operable overbroad temperature ranges, and are low cost due to the scalable approachto production. The printed gas sensors described herein can also operatein environments having a wide range of relative humidities and have ascalable, optimized signal to noise ratio that can be used to detect lowor high levels of a target gas.

The foregoing description of embodiments and examples of the inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the formsdescribed. Numerous modifications are possible in light of the aboveteachings. Some of those modifications have been discussed and otherswill be understood by those skilled in the art. While many of theembodiments herein disclose printed gas sensors that detect CO, thesensors and methods of manufacture described herein can be configured tomeasure other target gases such as H₂S, NO₂, SO₂, and the like. Theembodiments were chosen and described in order to best illustrate theprinciples of the invention and various embodiments as are suited to theparticular use contemplated. The scope of the invention is, of course,not limited to the examples or embodiments set forth herein, but can beemployed in any number of applications and equivalent devices by thoseof ordinary skill in the art. Rather it is hereby intended the scope ofthe invention be defined by the claims appended hereto.

What is claimed is:
 1. A catalyst ink composition for a printed gas sensor electrode comprising: a mixture comprising: about 67-79% of a metal catalyst; about 5-15% graphite carbon; and one of about 15% to 25% of a thermoplastic polymer powder; and a 3 mL ethylcellulose solution.
 2. The catalyst ink composition of claim 1, wherein the thermoplastic polymer powder comprises PTFE powder.
 3. The catalyst ink composition of claim 1, wherein the thermoplastic polymer powder comprises polypropylene powder.
 4. The catalyst ink composition of claim 1, wherein the thermoplastic polymer powder comprises polyethylene powder.
 5. The catalyst ink composition of claim 1, wherein the metal catalyst comprises Pt, Pd, Au, Ag, Ru, Ir, Co, Fe, Ni, C, or a combination thereof.
 6. The catalyst ink composition of claim 1, wherein the ethylcellulose solution further comprises octanol, isophorone, nonanol, deconol, or a combination thereof.
 7. The catalyst ink composition of claim 1, further comprising a surfactant.
 8. The catalyst ink composition of claim 7, wherein the surfactant comprises water, triton-100, carbopol, or a combination thereof.
 9. A catalyst ink composition for a printed gas sensor electrode comprising: a mixture comprising: about 67-79% Pt; about 5-15% graphite carbon; and one of about 22-25% PTFE powder, about 14-17% polypropylene powder, and about 14-17% polyethylene powder; and a 3 mL ethylcellulose solution.
 10. The catalyst ink composition of claim 9, wherein: the PTFE powder comprises a particle diameter of about 0.1 μm to about 5 μm; the polypropylene powder comprises a particle diameter of about 0.1 μm to about 5 μm; and the polyethylene powder comprises a particle diameter of about 0.2 μm to about 4 μm.
 11. The catalyst ink composition of claim 9, wherein the ethylcellulose solution further comprises octanol, isophorone, nonanol, deconol, or a combination thereof.
 12. The catalyst ink composition of claim 9, wherein the mixture comprises: about 74-79% Pt; about 8-9% graphite carbon; and about 14-17% polyethylene powder.
 13. The catalyst ink composition of claim 12, wherein the polyethylene powder comprises a particle diameter of about 2 μm to about 4 μm.
 14. The catalyst ink composition of claim 9, wherein the mixture comprises: about 74-79% Pt; about 8-9% graphite carbon; and about 14-17% polypropylene powder.
 15. The catalyst ink composition of claim 14, wherein the polypropylene powder comprises a particle diameter of about 5 μm to about 7 μm.
 16. The catalyst ink composition of claim 9, wherein the mixture comprises: about 67-71% Pt; about 7-8% graphite carbon; and about 22-25% PTFE powder.
 17. The catalyst ink composition of claim 16, wherein the PTFE powder comprises a particle diameter of about 1 μm to about 5 μm.
 18. A method of manufacturing a printed gas sensor, the method comprising: printing a catalyst ink composition onto a non-ionically conductive wick; heating the catalyst ink composition to a curing temperature for a curing period, thereby curing the catalyst ink composition and forming an electrode on the non-ionically conductive wick; and positioning the non-ionically conductive wick and the electrode within an electrolyte cavity formed by a substrate coupled to an encapsulation layer.
 19. The method of claim 18, further comprising loading an electrolyte into the electrolyte cavity.
 20. The method of claim 18, wherein, the catalyst ink composition comprises: a mixture comprising: about 67-79% Pt; about 5-15% graphite carbon; and one of about 22-25% PTFE powder, about 14-17% polypropylene powder, and about 14-17% polyethylene powder; and a 3 mL ethylcellulose solution. 